In a vacuum pressure swing adsorption process one or more adsorbents are used to adsorb one or more components of a feed stream and thereby produce a purified product stream. A typical process has a series of continuously executed steps in accordance with a repeating cycle. In the repeating cycle, an adsorbent bed containing the adsorbent is alternately used to produce the purified product and then is regenerated. During regeneration, the adsorbed components are desorbed from the adsorbent and then, the adsorbent bed is brought back into state in which it can be brought back on line and producing the product.
In a typical vacuum pressure swing adsorption process designed to make product oxygen from feed air, an adsorbent bed is subject to a seven step process conducted in the repeating cycle. For purposes of illustration only, such an adsorption process can be conducted with one bed. In a first step, the bed is simultaneously pressurized from the bottom with feed air and from the top with equalization gas delivered from a recovery tank. Thereafter, high purity product is added to the top of the bed from the oxygen surge tank while feed air is supplied by a compressor or other blower such as a Roots type of blower. In a third step, the bed continues to be pressurized from the bottom via the blower. The bed is now ready to make product and feed air is fed into the bottom of the vessel and product is removed from the top. The product gas is delivered to the oxygen surge tank. After production is complete, the blower is unloaded and the lower purity gas remaining in the top of the pressurized bed is transferred to the recovery tank. In a subsequent evacuation step, waste nitrogen is removed from the bottom of the vessel through the centrifugal compressor while there is no flow exiting or entering the top of the vessel. In the last step, the centrifugal compressor continues to remove nitrogen from the bottom of the vessel while oxygen purge gas is added to the top of the vessel. The pressure remains relatively constant during this step due to the fact that the oxygen purge flow is controlled equal to the evacuation flow. As would be known in the art, such a process could be carried out in multiple beds in which each bed is subjected to the steps outlined above.
As disclosed in U.S. Pat. No. 7,785,405, centrifugal compressors directly driven by direct drive high-speed permanent magnet motors have been advantageously utilized in vacuum pressure swing adsorption processes. The use of such motors allow for variable-speed operation such that the compressor and high-speed permanent magnet motor combination(s) can accelerate from low-speed to high-speed and decelerate from high-speed to low-speed rapidly, as required by the process. It has been found that this offers a major improvement over the use of centrifugal compressors driven by conventional induction motor/gearbox systems which due to the high inertia of the induction motor cannot accelerate and decelerate quickly. By continuously varying the compressor speeds to match the pressure ratio requirement for the compressor, which is varying because of the pressurizing and evacuating adsorbent beds, the centrifugal compressor used in such a cycle can be operated near, and preferably at, its peak efficiency from 100% design speed to a substantially lower speed.
Compressors are designed to operate within an operating envelope that can be plotted in what is referred to as a compressor map of pressure ratio between outlet pressure and inlet pressure versus flow rate through the compressor. On such a plot, a peak or best efficiency operating line is plotted in which for a given flow rate and pressure ratio, the energy consumption of the compressor is at a minimum. This compressor map can be programmed within a controller used in controlling the speed of the motor and therefore, the compressor. Depending upon the specific step in the vacuum pressure swing adsorption process, which would require a specific pressure ratio across the centrifugal compressor, the controller sends a signal referable to the optimal speed as determined from the compressor map to a variable speed drive that controls the speed of the high-speed permanent magnet motor.
There are, however, situations that can cause the compressor to move off the peak efficiency operating line and into a surge condition. For instance, there can be a lag in the control system, transitional steps in the process being conducted by the vacuum pressure swing adsorption apparatus, changes in ambient conditions and transitioning off the minimum speed line. In all of such situations, the mass flow being compressed can fall for a given speed and pressure ratio to drive the compressor into surge. A surge event is therefore, produced by a flow rate through the compressor falling below a minimum flow required at a given speed of the impeller of the compressor that is necessary to maintain stable operation. In a surge event, the head pressure developed by the compressor decreases causing a reverse pressure gradient at the compressor discharge and a resulting backflow of gas. Once the pressure in the discharge line of the compressor drops below the pressure developed by the impeller, the flow reverses once again. This alternating flow pattern has been found to be an unstable condition that can result in serious damage to the compressor impeller, drive mechanism and components. This condition must be avoided.
In repeating cycles employed in vacuum pressure swing adsorption apparatus, the operational conditions of the compression at which surge can occur will be most critical at high speeds. Additionally, during the evacuation and purge steps and particularly during the transition between the purge and evacuation steps, surge can occur quite unexpectantly. As will be discussed, the present invention provides a speed control that is particularly designed to avoid surge during low speed operation and during the evacuation and purge steps and the transition between such steps.